When light is reflected at an interface between materials of different refractive indices (for example, the interface between air and glass), the amount of transmitted light decreases so that the visibility may deteriorate. To prevent such reflection of light, using an antireflection film which has a moth-eye structure has been considered (see Non-patent Document 1 and Patent Documents 1 to 4).
The moth-eye structure is a minute structure whose size is generally equal to or smaller than the wavelengths of visible light (λ=380 nm to 780 nm). The effective refractive index of the moth-eye structure for light that is incident on a substrate continuously changes along the depth direction, from the refractive index of a medium on which the light is incident to the refractive index of the material that forms the moth-eye structure, whereby reflection of light is prevented. For example, in a moth-eye structure which prevents reflection of visible light, the two-dimensional size of the raised portions is not less than 10 nm and less than 500 nm. The incidence angle dependence of the antireflection effect which is achieved by such a moth-eye structure is small over a wide wavelength range. The moth-eye structure can be realized by a wide variety of materials. Also, the moth-eye structure can be directly formed on a substrate and therefore can be formed at a low cost.
The moth-eye structure can be formed by laser interference exposure or EB exposure. However, by utilizing anodization of aluminum, a large surface moth-eye structure can readily be formed at a low cost. Specifically, a porous alumina layer which is obtained by anodizing aluminum is used as at least part of a mold for the moth-eye structure, whereby the manufacturing cost can be greatly reduced. Thus, forming the moth-eye structure by means of anodization has been receiving attention (Patent Documents 2 to 4).
In this specification, the “mold” includes molds that are for use in various processing methods (stamping and casting), and is also referred to as a stamper. The mold can also be used for printing (including nanoimprinting). In the following descriptions of this specification, a mold which is used for formation of a moth-eye structure is also referred to as “moth-eye mold”.
Patent Document 2 discloses a mold which has a porous alumina layer formed by anodization over its surface. In general, recesses formed by anodization in the porous alumina layer, which are on the order of nanometers and which have cylindrical shape, are elongated in a vertical direction relative to the surface of the porous alumina layer, and are also referred to as micropores.
A porous alumina layer formed under specific conditions includes cells in the shape of a generally regular hexagon which are in a closest packed two-dimensional arrangement when seen in a direction normal to the surface. Each of the cells has a micropore at its center. The arrangement of the micropores is periodic. In a porous alumina layer formed under different conditions, micropores may constitute an arrangement with a regularity degraded to some extent or an irregular (i.e., non-periodic) arrangement.
Patent Document 3 discloses a mold which has tapered micropores with continuously changing pore diameters along the depth direction, the micropores having been formed by repeating anodization of aluminum and a pore diameter increasing process by means of etching. Patent Document 4 discloses a stamper (mold) which is manufactured by repeating anodization of aluminum and a pore diameter increasing process till adjacent holes are partially connected together.
FIG. 18(a) is a schematic perspective view of a stamper 700 disclosed in Patent Document 4. FIG. 18(b) is a schematic plan view of the stamper 700. At the surface of the stamper 700, there are six protrusions around each of a plurality of holes, and there are ridges extending between adjacent ones of the protrusions via saddle portions. In an antireflection element which is produced using the stamper 700 that has such a structure, the continuity of the effective refractive index at the interface between the antireflection element and the medium on which the light is incident (typically, air) increases so that high antireflection characteristics can be realized.